Image dissector



y 1940 P. T.,FAR,NSWORTH Re. 21,504

IMAGE DISSECTOR Original Filed April 26, 1933 FIG.|

villi!!!Ill'fllilfllllllll INVENTOR All! I I T U P T U 0 AMPLIFIEROSCILLATOR Reiuued July 9, 1940 UNITED STATES PATENT OFFICE macsmssnc'ron Philo T. Farnswortb, Fort Wayne, Ind., assignor,

by mesne assignments, to Farnsworth Television & Radio Corporation,Dover, Del., a cor- I porationof Delaware Original No. 2,087,683, datedJuly 20, 1937, Serial I No. 668,066, April 26, 1933. Application forreissue December 29, 1939, Serial No. 311,68

11 Claims. '(01. Inf-7.2)

- electrical impulses which may be retranslated to reconstitute thepicture.

Among the objects of this invention are: to provide a dissector tubehaving maximum sensitivity; to provide a tube wherein the entireemission of the photo-sensitive surface, over the entire time of use, isutilized to initiate the desired signals; to provide a tube of highvacuum characteristics, which does not depend upon ioni-. zationphenomena for its operation, and which therefore gives reproducibleresults and is highly stable in operation; .to provide a tube which iscathode rays, as in the hypothetical systems described byCampbell-Swinton.

Other objects of my invention will be apparent or will'be specificallypointed out in the description forming a part of this specification, butI do not limit .myself to the embodiment of my invention hereindescribed, as various forms may be adopted within the scope of theclaims.

- Referring tofthe drawing: Figure l is a longitudinal sectional view ofa television transmitting tube embodying this invention.

Figure 2 is a sectional view, on a greatly enlarged scale, of thecombined electron gun and target employed in the tube of Figure 1. V I

Figure 3 is a fragmentary elevational view of the electron gun andtarget illustrated inFigure 2, the shield surrounding this portion ofthe device being shown in section.

Figure 4 is a schematic diagram of the tube and its associated circuits.I Considered broadly the apparatus of my invention comprises aphoto-sensitive screen or plate a surface formed of a large number ofareas, or islandsf' of photoelectricallyemissive material. This screenis so positioned as to permit an optical image of the view to betransmitted to be projected thereon. An electron gun is positioned todirect a stream of cathode rays against the islands of photo-sensitivematerial, and means areprovideu I01 deflecting the stream to scan thesurface thereof, the stream being, preferably concentrated so as to forman extremely small spot at its point of impact with the screen.

The potentials within the device are so adjusted that the cathode raystream is decelerated as it approaches the plate, the velocity of theelectrons actually reaching the plate being extremely small. Inoperation, the photoelectric islands constantly lose electrons inproportion to the light intensity of the portion of the image fallingupon them, and consequently become constantly more positive. When theelectron stream is directed a ainst any individual island, the electronsneutralize the charge thus acquired. and the portion of the streamnecessary to effect this neutralization is therefore absorbed by thescreen. When the island under scansion has be come asnegative as themost negative portion of the electron gun, however, the electronsconstituent of the stream are brought to rest before they reach thescreen. directing this remaining portion of the stream into a workcircuit.

The electron flow in the cathode ray beam as it approaches the screen issubstantially constant.

When any particular portion of the screen is underscansion, the numberof electrons absorbed from the stream is equal to the total number lostby this portion due to photoelectric emission since last it was scanned.It follows that the lmabsorbed portion of the stream varies in intensityin accordance with the illumination of the portion of the screen whichit is scanning, thereby eflectively modulating the unabsorbed portionwith a picture current component.

Means are provided for In the preferred form of the device the stream ofcathode rays is electrically focused in the plane of the screen, and theelectron emission is reciprocally focused approximately in the plane ofthe electron gun. It follows that the unabsorbed portion of the streamis guided on its return path by the magnetic field in the same manner asthe photoelectric emission from the screen, and the photo-emission maythus be guided into a path other than the work circuit so that themodulated stream is undiluted by the emission from those portions of thescreen not under scansion; I

The general form of a preferred embodiment of my invention is shown inthe drawing. The envelope l is ,cylindricalin shape, one end of theenvelope forming a window 2 which is substantially plane. From the otherend of the The screen itself comprises a thin sheet ll of insulatingmaterial, which is preferably of mi'ca, but which may be of glass,enamel, or any other good insulator, preferably having a high dielectricconstant. In the screen,shown, the insulating layer is of mica, which issecured to the metal backing by clips l2.

The sensitive surface l4 which is carried by the face of the screencomprises an extremely large number of small, discrete insulated areasor islands of photoelectrically 'emissive material. Such a surface maybe prepared in a number of ways. In one method of forming the screen asheet of extremely fine wire gauze is placed over the face of the micasheet I I, and silver is evaporated on to the sheet through the gauze.When the gauze is removed there remains upon the mica the layer ofsilver covering those portions which were exposed through the openingsof the gauze,

separated by the uncoated lines where the micawas protected by thewires.

. With the screen thus prepared, when a tube is ready for evacuation itis pumped and baked in the usual manner, and is then filled withelectrolytic oxygen while hot and allowed to cool with the oxygen in thetube. A definite amount of caesium is then distilled into the tube,which is heated to a temperature of approximately 220 C. Readings of thephotoelectric current are then made, and at a definite point the tube isallowed to cool. If the correct amount of caesium has been admitted tothe tube, no short circuiting of the silver islands occurs, but if anexcess of caesium has been admitted small amounts of oxygen are admittedinto the tube, spoiling its sensitivity. The sensitivity recoversgradually and the process is then repeated until the shorting of theislands is "eliminated. With the proper amount of caesium admitted inthe first instance, its condensation occurs only on the silver islands,and the insulation elsewhere within thetube remains substantiallyperfect.

Another method of forming a screen of the required character comprisesfirst coating the mica with a powder of high insulating characteristics,such as powdered willemite. may then be evaporated directly onto theroughened surface thus formed, and substantially no shorting will 'occurbetween the silver particles deposited on the powdered surface. This isevidenced by the fact that the same amount of silver which, evaporatedonto .a smooth surface, gave a resistance of approximately three ohms,when,

evaporated onto the powdered surfacegave a resistance of 5x10 ohms. Theremaining treatment of this screen may be the same as that given to thegauze-formed islands.

In the other end of the tube, adjacent the window 2, there is formed alaterally projecting stem is. This stem surroimdsand supports a tubularfinger I which projects inwardly to the axis of the envelope, and iscushioned from the stem I! by a ring l1 of wire gauze. A lead I! isconnected to the shield and sealed out through the stem. An aperture 20is formed in the end of the finger facing the screen.

- Behind the aperture and directedthrough it is The silver backing anelectron gun comprising a circular anode 2| perforated with an axialbeam-canal 22, behind which is located thermionic cathode 24. Anode and.cathode are supported by leads 25, 25, and 21, which are sealed throughthe ends of a tubular glass support 29 fitted within the shield Hi. Theends of the leads are sealed out through the stem I5. Electron flowbetween the cathode 24 and the wall of the finger I6 is prevented by asmall cup-shaped shield 30, supported by the cathode lead 21, andpreferably maintained at the potential of the most negative portion ofthe cathode. 3] of fine mesh metal gauze, and the end finger is closedby a cap 32. I I

The connections of the device are shown in the diagram of Figure 4. Thecathode 24 is heated to emitting temperature by current supplied from abattery 35, regulated by a rheostat 36. A battery 31 or other source ofsupply maintains the anode 2I' at a potential positive to the cathode,preferably from 20 to volts, connection to the anode from the batterybeing made through the output resistor 39. A lead 40 connectswith' thelead 6 and so to the plate l0 which backs the The aperture 20 is coveredby a screen maintained at cathode potential, and a high poof the tentialsource 4| is connected between the cathode and the finger I6, throughthe lead [9, maintaining the finger from 500 to 1,000 volts positivewith respect to the cathode.

Surrounding the entire tube which is supplied with a constant directcurrent from the battery 44, this current being adjustable by means of arheostat 45. Placed on each side of the tube, with their axesperpendicular to the axis of the solenoid 42, is a pair of deflectingcoils 46, These coils are supplied with scanning current through anoscillator 41, which preferably generates a scanning wave of saw-toothform, and' at picture frequency -say 16 to 24 cycles in ordinarypractice, although'very much higher frequencies may be used if desired.A similar set of deflecting coils is also arranged perpendicular tothose shown, to effect the transverse scanning of the picture field, butas these deflecting coils and the oscillator which supplies them differfrom those shown fundamentally in frequency only, they are omitted fromthe drawing in order to prevent confusion. The frequency of this seconddeflecting system may be from 200 to 500 times that of the oscillator 41in ordinary practice, giving from 200 to 500 scanning lines in theresultant picture.

In operation, the image of the view to be transmitted is projected uponthe photo-sensitive surface H by a suitable lens system, typified in theschematic diagram by the lens 50. As a result, the various islands orparticles of photo-sensitive material at once begin to emit electrons inproportion to their illumination, these electrons being at onceattracted to the finger l6 by the high voltage field. Owing to the smallamount .of such photoelectric emission, and to the high voltageemployed, .saturation conditions exist, and no effective space charge isbuilt up adjacent the screen l4. The loss of electrons causes each ofthe discrete particles forming the surface to acquire a positive charge,but owing to the relatively large capacity between the particles and theback-plate I0 the acquisitionof this charge is accompanied by a verysmall rise in potential, in operation the difference in potentialbetween the particles and the back-plate being a few volts at most, evenunder the most extreme conditions.

is a solenoid 42,-

The electron streams fromthe particles are focused by the magnetic fieldof'the solenoid l2intheplaneoftheaperture2l,forminganelectrlcalimageinthisplaneashasbeend'escribedin my previous application, Serial No. 270,673,

only a relatively small portion of the electrons constituting the imageentering through the gauze II to strike the anode 1|.

Riverting now to the 'electron gun, electrons from the cathodeareaccelerated by the anode, the greater portion of them passing throughthe beam canal, 22 to form a beam of cathode rays having a velocitycorresponding to 50 volts. Upon emerging from the beam canal the streamof rays thus formed is further acceleratedto a 1,000 volt velocity bythe potential of the gauze II, and enters the body of the tube travelingat this high velocity. As the stream continues toward the sensitivescreen, however, it is decelerated by the potential gradient throughwhich it is traveling, arriving at the surface of the screen with nearlyzero velocity, this velocity depending upon the potential of theparticular photo-sensitive islands against which it is directed.

Electrons from the beam at once neutralize the positive charges upon theislands, b i n them substantially to zero potential, after which theremainder of the electrons are repelled, and, as they are within thefield of the finger; are at once accelerated in the reverse directiontoward the finger.

The main velocity of the stream ofcathode tion of the stream which isrepelled by the screen.

is subjected to conditions which are almost exactly similar to those ofthe photo-emitted radiation from the screen, and is therefore alsofocusedin the plane of the aperture.

A like reciprocal relation also holds with regard to the deflectingfield, that is, the cathode ray beam is directed by the focusing anddeflect field toward thatportion of the sensitive screen whose emissionis simultaneously being directed through the aperture, and that portionof the stream which is not neutralized or absorbed by the charges on thescreen is returned along almost the same path back to the finger andthrough the aperture 20. An extremely small portion is, of course,intercepted by thegauze II, but the remainder passes on and strikes theface of the anode 2|. Since this portion is-traveling against thepotential gradient between the gauze and the anode it is decelerated, soits velocity of impact against the anode is only that due to the voltagedifference between anode and cathode. By control of this potentialsecondary emission of electrons may be either. promoted or suppressed,and hence either primary or secondary emission may be utilized toestablish the picture current. i

The operation of the apparatus will now be clear. Each particle on thesurface of the sensitive screen is constantly emitting electrons, and

. comment.

thereby acquiring a positive charge. At 'the sametime the streamofcathode rays is scanning the surface of thescreen. When the streamarrives at the screen it neutralizes positive charges on thoseparticular islands which are under scansion at the moment, leavingthemneutralized portion to return to the' anode, the neutralized orabsorbed portion of the rays being substantially equal to the totalnumber of electrons emitted .by the' islands since last they werescanned. Since the scanning intervals-are equal, the variations in thereturn' beam are directly proportional to the variations in the emissionof the successive particles that=the beam traverses, integrated over theentire period between scansions.

Those electrons reaching the anodepas through the output resistor I! andthence back to the cathode, completing the circuit. An amplifler Si iscoupled across the output resistor through the blocking condenser 52,the amplifier feeding any conventional'type of output circuit,

- such as a line or radio transmitter.

Several features of the device deserve further It is to be'noted, forexample, that the apparatus is fully' operative if the metal backplatel0 ofthe sensitive screen be omitted. The plate is desirable in ordermore quickly to establish operating conditions, and to reduce thepotential differences effective on the screen, but even if it beomitted, scansion of the screen by the cathode ray beam will eventuallycause the accumulation of a negative potential sufilcient to bring thestream to rest and repel it, after which the loss of electrons andneutralization of a portion only of the beam will occur in mannersimilar to that obtaining when the back-plate is present. Furthermore,inequalities in the insulating layer are not important. It is the totalnumber of electrons lost by the individual islands which determines theproportion of the stream which is absorbed, and not the potential gainedby the loss of those electrons. Hence a difference of several hundredper cent in capacity as between the various islands has no materialefifect. The chief value of the back-plate is to potentials andpotential differences, thus reducing leakage through the insulator to aminimum, and causing minimum aberrations in the magnetic focus due tovariations in velocity of the electron stream at its point of impact.

The fact that the photo-electrons emitted by the screen are guided bythe focusing field and,

with the exception of a small area surrounding that immediately underscansion, are prevented from reaching the anode. is valuable in that itprevents variable dilution of the return stream which would mask itsmodulation. A constant amount of dilution of the stream is notimportant, since the blocking condenser 52 filters out 'the entiredirect current component, and it is however, this is not found to bejustifiable in view of the additional complication involved, slight 70though this may be. e

I am aware that many previous efforts have,

been made-to accomplish scansion by means of a stream of cathode rays,but although various methods of so doing have been proposedthey u haveall differed fundamentally from the present invention. ThusjSchoultz, inhis French patent, has shown the use of a cathode ray stream to scanaphotoelectric surface, utilizing increased secondary emission ofphotoelectric materials when under .illuminationto develop his picturecurrent. In my invention the velocity of the stream as it approaches thescreen is so low that substantially no secondary emission of electronsoccurs, and the number of electrons returning from the screen isdecreased with increased illumination instead of being increased therebyas in Schoultzs device. V Other inventors have used space chargesestablished by photoemission to modulate an electron stream passingtherethrough, whereas in my invention no space chargesare establishedand actual' absorption or neutralizationof the electrons forming thestream causes modulation. Still other inventors have used an electronstream to establish a conducting path "between isolated islands ofphoto-emissive material and a backplate, relying on the liberation ofspace charges through the establishment" of such a path for theirpicture current, the picture current being formed entirely fromphoto-electrons or from ionization caused by their liberation. In thepresent invention the photo-electrons themselves.

do not enter into the picture current, which is negative in sign incomparison to a direct photoelectric current, no physical conductionoccurs to the back-plate, and ionization is rigorously avoided.

The tube is remarkably stable in operation due to the factors alreadymentioned. Furthermore, the entire photo-emission of the screen, duringthe entire time, is effectively utilized. Were direct photo-emissionused, each elementary area would be active .to produce the picture onlyduring the period when it was under scansion. In a 320 line picture,this period would be aprox-. imately one one hundred thousandth of thetotal time, its emission during the rest of the time being wasted. Inthe present invention its emission during the time when it is not underscansion is effectively stored, and its integrated effect is madeavailable at the instant when it is contacted by the beam. The remainingtheoretical gain in efficiency is a factor of one hundred thousand.Actually, owing to the fact that portions of the screen arenon-emissive, and that a portion of the return stream is absorbed by thegauze 3| the factor is not this great, but it is still large incomparison with tubes of other types.

, I claim:

1. The method ofgenerating a signal repre senting a picture whichcomprises the steps of forming an optical image of the view to betransmitted and projecting said image on a photoelectric surface to forma photoelectric discharge, said discharge forming a charge image oversaid surface corresponding to the optical image focussed thereon,accelerating said discharge in charge image, and utilizing my effectsproduced discharge from the element of said surface under scansion at agiven instant is substantially undiluted by discharges from otherelements of said surface, and collecting only such electrons as aretraveling away vfromsaid surface from the direction of the point'underscansion by said beam to form a signal current.

3. The method of scanning an image on a photoelectric surface whichincludes the steps of directing a stream of cathode rays toward saidsurface, decelerating said stream by the charges accumulated on thesurface from the stream, accelerating that portion of the stream notcontributing to said charges to form a reverse stream, and deflectingsaid first stream to scan said surface with the first stream and therebymodulate the reverse stream with a picture signal component, andcoordinately deflecting said reverse stream along a path substantiallyparallel with said first stream.

4. The method of producing a signal current representing a picture to betransmitted which includes the steps of establishing a surface of basepotential, photoelectrically discrete positive charges forming a patternde: pendent on the picture to'be transmitted in a second surfaceadjacent and substantially parallel to said base potential surface,liberating a sub-- stantially constant supply of electrons at a positionspaced from' said surfaces and substantially at said'base potential,accelerating said electrons by a potential materially above said basepotential to form a clearly defined stream of cathode establishing raysdirected against said discrete charges, scan,-

ning said discrete positive charges with said stream of cathode raystodecelerate to zero velocity a portion of said stream dependent uponthe charges against which it is directed, reaccelerating saiddecelerated portion of said stream in a new direction to form a secondclearly defined stream modulated in accordance with said discretepositive charges, and utilizing the effects of said scanning .of saiddiscrete positive charges to develop a picture signal.

5. A transmitter of television signals comprising a screen of insulatingmaterial having discrete areas of photoelectrically emissive material onthe face thereof, means for projecting an optical image of a scene to betransmitted on said face of said screen to establish discrete positivecharges thereon corresponding to said optical image, an electron guncomprising a cathode and a perforated anode positioned to direct astream of cathode rays against said face, means for deflecting saidstream to scan said screen, and

. means for redirecting portions of said stream unabsorbed by saidscreen in the direction back to said anode, and means for utilizing theeffect of the scanning of said screen to develop a picture signal.

6. A transmitter of television signals comprisa screen of insulatingmaterial having discrete areas of photoelectrically emissive material onthe face thereof, means for directing a concentrated stream of electronsagainst said face-with a velocity approachingzero at the point of impacttherewith, and means for directing electrons approaching and leavingsaid screen in oppositely directed predetermined paths substantiallycoincident throughout their length.

'1. A transmitterof television signals comprising an evacuated envelope,a screen of insulating material within said envelope having discreteareas of photoelectricaliy emissive material onthe face thereof, anelectron gun positioned to direct a stream of cathode rays against theface of said screen, and an electrostatic shield enclosing said electrongun and provided with an aperture to permit the passage of electronsbetween said gun and screen.

8. A transmitter of television signals comprising an evacuated envelope,a screen within said envelope adapted to receive an optical image of thepicture to be transmitted on the face thereof and comprising aninsulating body, a conducting back, and a photosensitive face ofdiscrete elementary areas of photoelectrically emissive material, anelectron gun within the envelope positioned to direct a stream ofelectrons against the face of said screen, means for directing electronsemitted from said screen back toward said electron gun, and means forpreventing electrons from all of said screen with the exception of alimited area from reaching said electron gun.

9. A transmitter for television signals comprising an evacuated envelopehaving a window therein, a screen comprising insulated islands ofphotoelectrically emissive material positioned to receive an opticalimage projected through said window, an electron gun within saidenvelope comprising a cathode and an anode positioned to direct a streamof cathode rays against said islands, an electrostatic shieldsurrounding said gun and apertured to permit the passage of said cathoderays, means for deflecting said stream to scan said screen, means forestablishing a magnetic field between said electron gun and said screento guide said stream to said screen and return the portion of saidstream unabsorbed by the screen through the aperture in said shield, andmeans for utilizing the portion of said stream returning through saidaperture to initiate a signal current.

10. A transmitter for television signals comprising an evacuatedenvelope having a window therein, a screen comprising insulated islandsof photoelectrically emissive material positioned to receive an opticalimage projected through said window, an electron gun within saidenvelope comprising a cathode and an anode positioned to direct a streamof cathode rays against said islands, an electrostatic shieldsurrounding said gun and apertured to permit the passage of said cathoderays, means for deflecting said stream to scan said screen, means forredirecting a portion of said stream unabsorbed by said screen throughthe aperture in said shield, and means,

for utilizing the portion of said stream returning through said apertureto initiate a signal current.

11. The method of scanning a photoelectric surface to produce a picture.signal which comprises developing a charge image on the surface,corresponding to an optical image to be transmitted, developing a beamof electrons and directing said beam at the surface, decelerating saidbeam by the charges accumulated on the surface from said beam, scanningsaid charge image with said beam to vary in accordance with said chargeimage the portion of said beam utilized to accumulate said charges,accelerating .that portion of said beam not. contributing to saidcharges to form a reverse clearly defined stream of electrons, causingthe reverse stream substantially to retrace the path of said beam, andutilizing the effect of the scanning of said charge image to develop apicture signal.

PHILO T. FARNSWORTH.

